1. Field of the Invention
The present invention relates to refrigeration systems and more particularly relates to a high efficiency refrigeration system employing an improved evaporator design which may be utilized in conjunction with sub-coolers and the like, to form an improved system.
2. State of the Prior Art
By way of background, a refrigeration system uses a refrigeration cycle which is employed in refrigerators, heat pumps and air conditioners. The refrigerator becomes a heat pump when it is used to produce a heat flow into or out of a building. When it causes a heat flow out of the building it is then also called an air conditioner. As shown in the background diagram of FIG. 1, a refrigeration system 10 includes a condenser 12, a throttling or expansion valve 14, an evaporator 16 and a compressor 18. The refrigerant flows in either a gaseous or liquid state (sometimes a mixture of the two) by way of lines or piping, the direction of the flow being as indicated by the arrows 8. In the ideal refrigeration cycle, schematically illustrated in FIG. 2 the pressure (P) vs. volume (V) of refrigerant saturated liquid refrigerant passes through a throttling or expansion valve 14 and the liquid expands into a gas with some entrained liquid as shown at "b". The gas, with a mixture of liquid passes through the evaporator 16 which, in the case of a refrigerator, allows heat to be removed from food stuffs and the like and transferred to the gas, liquid mixture. As the gas picks up heat it expands and the volume increases. The gas is then compressed by the compressor 18 as illustrated in lines "d-e" in FIGS. 1 and 2, and then passed through a condenser 12 which gives off heat as the volume of the gas decreases and the pressure remains substantially constant. In the ideal refrigeration cycle, the compression from d to e is adiabatic and the gas increases in pressure with a decrease in volume. Moreover, because of the expansion through the throttling or expansion valve from a saturated liquid at "a" to a gas liquid mixture at "b", the process from "a" to "b" is also considered to be adiabatic. The coefficient of performance of an ideal refrigeration system is depicted as a heat flow in "b-d" (i.e., through the evaporator 16) relative to the work added by the compressor. Inasmuch as the work added by the compressor must equal (in an ideal system, excluding losses) the heat flow from states "e" minus the heat flow in "b-d", it is apparent that the coefficient of performance will increase by an increase in the heat flow in the portion of the cycle "b-d" relative to the amount of work added to the system by the compressor. Thus, for example, the more efficient the evaporator for the same amount of compressor work, the higher the coefficient of performance of the system.
With respect to freezing food stuffs in the evaporator, for example in the manufacture of ice cream, ice, non-dairy confectionery products, (hereinafter soft serve desserts) and the like it has been common practice to employ a scrapped surface heat exchanger constructed by surrounding a stainless steel cylinder with a refrigeration jacket. The refrigerant, in such a construction, is in direct contact with the stainless steel cylinder thereby cooling the surface of the cylinder such that the food stuff is quickly frozen and then scrapped off the inner freezing cylinder surface. The construction of this kind of refrigeration jacket is somewhat complex and involves a substantial amount of welding which includes internal baffles for directing the refrigeration flow.
A more common method of constructing the freezing cylinder is to wrap the stainless steel with copper tubing and route the refrigerant through the tubing to thereby transfer the heat from the freezing cylinder to the refrigerant through the tubing. For example, the Clancy patent, U.S. Pat. No. 2,364,130 illustrates as heat exchange apparatus with tubing in intimate contact with and spirally wound about a tubular shell or sleeve. One of the difficulties in this construction method is ensuring a good attachment of the copper tubing to the stainless steel in a manner which affects good heat transfer. Another attempt at improving the performance of the evaporation of the freezing cylinder is illustrated in U.S. Pat. No. 5,419,150 issued on May 30, 1995 to Kaiser, et al. who attempted to improve performance of the evaporator (freezer) by increasing the surface area on the interior of the evaporator by utilizing an additional inner refrigeration tube (See FIG. 12 of Kaiser).
Other means for increasing efficiency of heat exchangers are illustrated in the prior art such as Evinger, U.S. Pat. No. 1,837,416, wherein a heat exchanger is illustrated with a suitable length of helically wrapped metallic sheeting and an inner corrugated shell formed of such material as a copper or copper alloy. The Sutter patent U.S. Pat. No. 1,514,877, the Bowling patent U.S. Pat. No. 2,611,585 and the Huggins patent U.S. Pat. No. 3,486,489 all illustrate different constructions of heat exchangers. None of the prior art illustrates the invention as provided in the claims of this application. In U.S. Pat. No. 4,896,247 to Cozer, a cooling fluid flows through spiral passages around the inner chamber to draw heat from the contained vision system. Cozer recognizes that the spiral flow pattern enhances the cooling efficiency of the cooling system. However, Cozer does not recognize the need for increasing the surface area exposed to the refrigerant so as to enhance and maximize efficiency.
It has been discovered that the most important factor in heat exchange in this kind of evaporator is the transfer of heat from the evaporator itself to the refrigerant. By dramatically increasing the surface area to which the refrigerant is exposed, heat is removed faster and more efficiently from the freezing chamber to the refrigerant.
Of course there are other means to increase the efficiency of the evaporator so that it may be made more compact and more efficient while still allowing the passage there through of increased volumes of food stuffs such as soft serve desserts.
Sub-coolers in a refrigeration system are well known devices which are employed to evaporate low pressure liquid refrigerant which has not fully changed state to gas in the evaporator so that the liquid does not return to the compressor where it might cause damage. Typically, sub-coolers in a refrigeration system serve this purpose by providing a heat exchanger between the high pressure liquid refrigerant and the cold, low pressure suction line refrigerant gas. The condition which is inhibited is called refrigerant "flood back". Refrigeration systems such as an ice machine, which employ a defrost cycle, are typically fitted with sub-coolers.
It has been found that, in conjunction with the evaporator of the present invention, the addition of a sub-cooler allows full utilization of the evaporator heat transfer surface under a wide range of operating conditions. For example, without a sub-cooler it has been necessary to design the evaporator for about a 60% utilization under nominal conditions so that there is no excessive flood back during severe conditions. This margin of safety has been found, in the past, to necessitate a larger and more expensive evaporator with an increased volume of food stuffs (e.g., soft serve desserts) subject to product recall due to break down because of a longer holding time in a frozen state.
With respect to sub-coolers, such U.S. patents as Newton, U.S. Pat. No. 2,120,764; Brown U.S. Pat. No. 3,852,974; and Woods U.S. Pat. No. 4,696,168 all illustrate the advantages of a sub-cooler in a refrigeration system, i.e., to inhibit flood back. None of them illustrate the advantages of utilizing an evaporator of high efficiency construction such as shown and taught by the present invention.